In a conventional embedded servo positioning system of a disk drive, the detection of off-track positioning of a read/write head may involve converting position bursts of a filtered, differential disk drive read signal into a normalized voltage level output proportional to the relative head/disk position. Typically, the generation of this normalized position error signal and off-track position detection requires the use of servo off-track detection circuitry which is distinct from the existing servo track position detection circuitry. This circuitry may include compare and select circuits to determine servo burst selections, a programmable DAC to set a predetermined threshold level defining the boundary between on-track and off-track position, sum and difference amplifiers to form the sum and difference signals of the normalized signal and a high resolution comparator to compare the normalized analog signal to the reference or threshold level. The result of this comparison produces a logic output which determines if the relative head/disk position exceeds the threshold level. This logic output signal may then be used to gate writes to the disk.
The primary disadvantage associated with such an approach is that the additional circuits or devices required for off-track position detection increase the size, complexity and power consumption of the track position detection circuitry while disk drive design objectives continue to target improved performance and smaller form factors.
A need, therefore, exists to provide an improved apparatus for off-track position detection that utilizes existing types of track position detection circuits having charge redistribution A/D circuits. It is desirable to allow these charge redistribution A/D circuits already used by a disk drive control system to digitize the normalized burst signals for servo tracking to also perform the off-track position detection function.